1. Field of the Invention
The present invention relates generally to the very low-power detection of the resistive or the resistive-capacitive characteristics of certain substances, such as fluids which are either liquid or vaporous, and more particularly to leak or spill detection apparatus for sensing the resistance, or the parallel capacitance and resistance, of a substance as an indication that a leak or spill of the substance has occurred, with one or more remote probes being connected to a single display and alarm unit.
2. Brief Description of the Prior Art
Inert materials, such as air or deionized water, have an extremely high resistance, but are not hazardous or corrosive and in use occasionally require detection by apparatus of the type toward which the present invention is directed. However, hazardous or corrosive fluids, which contain large amounts of ionized material that can combine corrosively or in a hazardous way with other matter in their environment often require detection. Such materials have a relatively low resistance because of ionic conduction and are difficult to monitor or detect.
There are many applications in which it is important that a leak of a hazardous or corrosive fluid be detected as soon as possible after it has occurred. Some of these applications involve corrosive and/or combustible fluids that will explode in the event a spark is generated. Others involve electrolytic but non-explosive fluids that can be detected by simply detecting the ionization process across a pair of electrodes.
In one embodiment of the prior art, a leak detection apparatus is shown in U.S. Pat. No. 5,190,069 for monitoring leakage from household water systems. Upon detection of a leak, the water supply valve is automatically shut off. A pair of spaced apart wires are imbedded in an insulating tape with liquid cell sensor elements formed at spaced intervals about uninsulated sections of the wires. The tape is placed about the pipe to be monitored so as to collect the leaking water within the liquid cell sensor elements to electrically connect the conductors within a cell. Electronic circuitry actuates a servo to turn off a supply valve and/or sound an alarm. The water collecting elements eliminate unintended alarms due to condensation.
A prior art leak detection and shut-off apparatus for preventing damage from leaking hot water tanks shown in U.S. Pat. No. 5,334,973 features a unique three-layered sensor that completes encases the hot water tank thus detecting the occurrence of leaks anywhere on the tank. Circuitry controls the sensor that provides for accurate detection of even small amounts of moisture so that any potential leak problems can be remedied before a major spill occurs. The device also features a water shut off control mechanism plus audible alarm in the event a leak is detected. The unit is powered by connecting it to 110 house current. A 9 volt battery back-up is provided in case of power failures.
Another prior art device shown in U.S. Pat. No. 5,150,603 is a hydrocarbon vapor sensor which is simple and inexpensive and can easily detect the existence of hydrocarbon vapor. The device is temperature compensated so that a change in resistance due to a change in temperature is avoided and an accurate response is always provided. The sensor can be used in a system in which a number of sensors are provided at various depths to determine whether a leak or a ground spill has occurred.
A battery-powered, liquid-detection alarm and shut-off system including an elongated sensor, an electronic controller, and a spring biased shut-off valve assembly is shown in U.S. Pat. No. 5,008,650. The sensor comprises two partially exposed conductors separated by protruding ridges of insulation. The protruding-ridge configuration enables the sensor to detect small quantities of surface water and yet be insensitive to humidity. The electronic controller maximizes battery life and protects against false alarms caused by electrical interference. The shut-off valve assembly comprises a 1/4-turn ball valve and a valve-actuating assembly. The valve is set by being manually cocked to an open position, engaging a lever actuated cam and preloading a valve shut-off spring. The valve is then closed by momentarily passing an electrical current through a temperature-activated memory-shaped spring located on the valve assembly. The current is supplied by the electronic controller in response to detection of liquid by the sensor. The heating of the spring from the induced current causes the tension of the memory-shaped spring to increase, releasing the lever actuated cam and permitting the preloaded valve shut-off spring to close the valve.
U.S. Pat. No. 4,843,327 describes a detection and location system, e.g. for liquid leaks, comprising a trunk line sensor cable and at least one branch line cable; each of the cables comprise two insulated conductors and two non-insulated conductors which are not connected to each other in the absence of a leak but which become connected upon occurrence of a leak. In the trunk line cable, the insulated conductors form part of a circuit which enables the location of a leak to be detected. In the branch line, one of the insulated conductors and one of the non-insulated conductors are connected so as to form a loop connecting the ends of the non-insulated conductors of the branch cable.
As shown in U.S. Pat. No. 4,710,353, a detector for detecting leaks of a corrosive liquid such as strong acids or bases is provided comprising a light guide core having a covering which generates heat upon contact with the liquid to be detected. The heat changes the light transmissivity of the light guide, which change can be measured. The covering comprises a porous polymer having a salt within its pores, such as an ammonium salt, which dissolves in the liquid to be detected and generates heat.
In U.S. Pat. No. 4,677,371, coaxial cable and a bare wire are aligned in parallel relationship and affixed within insulation covers which have openings therein so as to partly expose both the bare wire and the outer conductor of the coaxial cable to the atmosphere, thereby forming a leak-detecting sensor. By the use of resistance meters, the resistance of the core wire of the coaxial cable is monitored and utilized to detect both the presence and the location of a leak.
As disclosed in U.S. Pat. No. 4,386,269, to detect leaks from pipelines carrying fluids, especially oil, light is transmitted through a fibre-optic held in proximity with the pipeline. The fibre-optic is surrounded by a medium of which the refractive index is altered by the influence of the leaked fluid. In a preferred embodiment the medium is a silicone rubber of which the refractive index is normally lower than that of a quartz fibre optic, but of which the index increases to that of the quartz or above when oil soaks into it through a permeable cladding and an elastomeric protective layer, thus rendering the fibre optic non-internally-reflective so that light is absorbed. Control means linked to a light receiver detect that change and the position of the leak is located to within the length of the optic. In another embodiment, the medium is a liquid which is expelled from around the optic by the action of leaked fluid.
In another embodiment of the prior art, a first light beam generated from an electrical emitter (LED) is directed through a translucent material to a 45.degree. wall in said translucent material, such that the light is reflected through the translucent material to a second 45 .degree. wall, which is at a 90.degree. angle to the first wall, so that the light is again reflected back in the direction from which it came, and can be detected by a light sensing element near the emitter. If a substance, such as a liquid, comes in contact with the first and the second walls, the light is substantially refracted into the substance, and the intensity of the reflected light is greatly reduced; assuming the liquid is itself translucent. The reduction in the intensity of the reflected light is then indicative of the presence of the substance or liquid. This is then indicative of a leak condition.
In still another embodiment of the prior art, capacitance of a substance between parallel conductors is measured. Since the substances are presumed to have a dielectric constant different from the air normally between the parallel conductors, a difference in capacitance indicates the presence of the substance being monitored.
Several limitations of the prior art are known:
With electro-optical detectors, the reflective walls forming the 45.degree. light reflecting angles must be kept very clean and the material must be translucent, with a high refractive index to the substance to be detected, to allow the refraction of light in an amount that will permit detection of the substance being monitored. This implies that considerable maintenance must be performed to keep the detector clean and in good working order, especially in a corrosive or nonresidue-depositing fluid environment. Moreover, the materials used typically have relatively poor resistance to corrosion from many of the chemicals that are monitored. The materials that must be used generally limit the maximum temperature within which they may be used (no more than 80.degree. Centigrade) to avoid losing clarity and integrity of the shape. Further, such embodiments cannot detect very small amounts of leakage, are not very accurate, and are orientation sensitive. Another disadvantage is that normally the light emitter must be located within the sensing element, thereby requiring that potentially unsafe levels of electric power be delivered in chemically hazardous or explosion-prone environments.
Similar problems are encountered with implementations of fiber-optic devices for detecting liquid spills. The fiber-optic devices are affected by the reflectivity of the substance, lack of resistance to chemical interaction with the emitter and sensor, plugging of the device cavities, and lack of sensitivity. Furthermore, in many applications the light beam cannot be directed horizontally across the substance being monitored because of problems with parasitic reflections from, for example, the spill sump.
Capacitance sensors also suffer from problems with parasitics; in this case parasitic capacitances from, for example, the wall of a container. The capacitive sensor relies at least in part on actuation by the mass or density of the materials, which limits its sensitivity to the extent that it is not suitable for the detection of small amounts of leakage. Also, the capacitance which indicates the presence of a substance being monitored must be measured at high power levels for reliable results. High power levels are intrinsically unsafe in some environments. Further, fringing effects of the electric fields increase the likelihood of parasitic capacitances from other materials having an adverse effect or even causing a false actuation, and of interference with sensitive electronic equipment that may be nearby.
Previous detectors utilizing the resistive characteristics of the substance being measured have typically operated at high power levels, or have created possibly dangerous or deleterious conditions due to heating of the substance being monitored, the potential of a spark being generated, or a substantial and unsafe amount of leakage. Such devices have thus largely been confined to the detection of water or other inert materials. Further, the prior art does not teach the detection of very thin layers; such as 0.001 inches or better, which can be measured by this invention. In fact, the prior art teaches the detection of 0.25 inch or more layers; which is thought to be the lower limit for prior art methods.